1. Field of the Invention
This invention is in the field of token-passing local-area networks and more particularly relates to an improved method by which one module of such a network passes a token to another specified, or addressed, module, which token, when accepted by the addressed module, authorizes that module access to the medium of the network for the purpose of transmitting information over the medium.
2. Description of the Prior Art
In any local-area network where a single communication medium is shared by many modules, there must be an access control mechanism or procedure which provides each module with access to the medium for the purpose of transmitting information. Some systems allow collisions; i.e., several modules can transmit simultaneously and provide methods for resolving such unacceptable conditions so that only one module at a time has access to the medium for the purpose of transmitting data. Token-passing access control methods allow a module which has accepted a token from another module to have exclusive access to the medium to transmit information to other modules normally for a limited period of time, at the end of which period the module having the token must transfer a token to another successor module.
To do this, a special frame, a series or set of binary digits called a token pass frame, or a token, is transmitted from one module to another module around a logical ring based on the physical address assigned to each module. While a module has access to the medium, it is permitted to transmit information, an information frame, to one or more modules connected to the medium before passing a token to a success or module. An advantage of token-passing methods of controlling access to the medium of the network is the determinism of such methods. The number of modules comprising the logical ring of the network is known. From this information, it is possible under non-error conditions to calculate the maximum time it will take for a given module to receive a token addressed to it by a predecessor module, or, stated another way, the maximum time it takes to pass a token to each module of the ring, or to complete a ring cycle. With other access-control mechanisms, it is only possible to determine within a specified time period that there is a given probability that a given module will be given access to the medium for the purpose of transmitting information. This deterministic characteristic of a token-passing local-area network is important, particularly in the operation of process-control systems during process upset conditions.
A problem with token-passing methods of the prior art networks is that when additional modules are incorporated into the network or when modules are removed or become non-operative, some special action is required to cause the network to incorporate additional modules or to delete those removed or nonfunctioning with a minimum of undesired consequences to the network.